Modern photolithographic semiconductor fabrication processes produce integrated circuits on a semiconductor wafer by forming patterns in successive layers. One method of creating a layer is to form a layer of a desired material, form a photoresist layer on the material layer, expose light through a mask to impinge upon the photoresist, and develop the photoresist to create a pattern. The wafer is then etched, removing selected portions of the material underlying the photoresist corresponding to the pattern exposed through the mask. Thereafter, the exposed photoresist is removed, and another material layer is applied, followed by a photoresist as the formation process continues. As part of the pattern creation process, the wafer is often moved from one photolithography tool to another for the creation of the successive layers.
In order to ensure that a working integrated circuit results from the photolithography processes, various elements within each tool, for example, a chuck holding a wafer and a mask holder holding a mask, must be aligned to ensure each created layer is in the appropriate position relative to the previous layer. Generally, some level of misalignment occurs between the layers during the production process. Overlay control attempts to monitor and correct the misalignment between the various layers.
One of the most difficult areas to correct overlay misalignment involves leading lots, the initial group of wafers produced with a mask and tool combination. This is particularly true in automated manufacturing facilities where, in order to avoid leading lot problems, a wafer is often confined to the particular tool used to expose the initial layer. In an attempt to reduce overlay misalignment problems for subsequent layers, the mask is changed in the tool prior to exposure of the next layer, thus allowing formation of the next layer without having to realign the wafer. The frequent mask changes generate extra loading requirements that necessitate a human supervisor to correctly arrange each lot according to the tool used for the initial exposure. Human involvement and frequent mask changes that require alignment increase inefficiency and greatly increase semiconductor production time. Additionally, these automated systems cause process interlock that increases operation loading because the operator cannot visually identify the leading lot wafers.
Generally, fully automated semiconductor manufacturing methods do not record the overlay alignment values as a reference. Failure to record overlay alignment values introduces additional rework due to misalignment because the system cannot relate and adjust the present production process based on the experiences of past production processes. Therefore, there is a need for a system and/or method for improved automated semiconductor wafer manufacturing that addresses at least some of the problems and disadvantages associated with conventional methods.